Radio frequency power measurement

ABSTRACT

An unknown RF power is converted to a voltage by a detector ( 11 ). The voltage is converted to a number by a digitiser ( 12 ). A separate and fixed RF power reference ( 14 ) drives a second detector ( 15 ), whose voltage output is also converted to a number by the digitiser ( 12 ). The RF detectors are thermally coupled. Temperature variations in the RF power to voltage characteristic of the detector ( 11 ) are corrected by measuring the RF power reference ( 14 ) via the detector ( 15 ).

FIELD OF THE INVENTION

This invention relates to the measurement of radio frequency (RF) power.

BACKGROUND

FIG. 1 of the accompanying drawings is a block diagram showing a knowntechnique for measuring RF power.

The unknown RF power to be measured is applied to a detector 1, whichconverts the RF power into a more easily measurable quantity, such asvoltage or temperature (which may in turn be converted to voltage). Thevoltage representing the RF power is digitised by a digitiser 2. Toprovide a reading of RF power to the user of the equipment, the outputof the digitiser is usually scaled by a numeric correction function innumeric correction block 3, which takes into account the transferfunction of the detector, and any fixed offsets which would otherwisegive errors at low RF powers.

In a practical implementation of the technique described, the detectorblock is typically a diode followed by a capacitor. The transferfunction of this block, expressed as voltage out divided by RF power in,is a function of frequency, temperature, level and the batchcharacteristics of the diode. To avoid the user of the equipment havingto take these factors into account each time a measurement is made, thedetector and digitiser are characterised at initial manufacture usingknown RF input powers, and the results are retained in the storednumbers block 8. When the user makes a measurement of an unknown RFpower, the stored numbers are used by the numeric correction block 3 toscale the output of the digitiser 2, and hence give a correctedmeasurement.

The process of characterisation at initial manufacture using known RFinput powers is called calibration. FIG. 2 of the accompanying drawingsis a block diagram showing a known technique for calibration. Forsimplicity, it will be assumed that the detector is used in a regionwhere the voltage output is proportional to the power input, and thatthere are no offset errors.

A known RF calibration power RF_(calpower) produced in power source 9 isapplied to the detector 1 at a series of known frequencies Fn coveringthe range of interest. At each frequency the output of the digitisergives a value N_(1Fn), which can be used together with the known RFcalibration power to give the transfer function T_(1Fn) of the detector1 and digitiser 2 at each frequency Fn:

$T_{1{Fn}} = \frac{N_{1{Fn}}}{{RF}_{calpower}}$

This series of numbers T_(1Fn) is retained in the stored numbers block8.

Referring again to FIG. 1, when the user makes a measurement of anunknown RF power, provided the frequency is known the appropriate storednumber can be used by the numeric correction block 3 to scale the outputfrom the digitiser 2, and hence give a reading of the unknown RF power.

This technique takes into account the variability of transfer functionwith frequency and with the batch characteristics of the diode. It doesnot however taken into account the variability of transfer function withtemperature. If a measurement of an unknown RF power is made at adifferent temperature from the temperature at the time of calibration,significant errors result.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a RF powermeasurement system which at least alleviates the foregoing problem.

According to the present invention the variability in RF power readingwith temperature is significantly reduced by providing a second detectordriven by a constant RF power source. In effect, variations in thecharacteristic of the first detector with temperature can be compensatedfor by observing variations in the characteristic of the seconddetector.

According to one aspect of the invention there is provided a RF powermeasurement system comprising, a first detector for converting RF powerto be measured to a first measurement signal, a reference source of RFpower, a second detector, thermally coupled to said first detector, forconverting RF power produced by said reference source to a secondmeasurement signal, and processing means for numerically applying apredetermined correction factor to said first and second measurementsignals obtained at the same temperature to derive a value of said RFpower to be measured.

According to another aspect of the invention there is provided a methodof measuring RF power comprising the steps of using a first detector toconvert RF power to be measured to a first measurement signal, using asecond detector, thermally coupled to said first detector, to convert RFpower produced by a reference source to a second measurement signal, andprocessing said first and second measurement signals obtained at thesame temperature by numerically applying a predetermined correctionfactor to said first and second measurement signals to derive a value ofsaid RF power to be measured.

BRIEF DESCRIPTION OF DRAWING FIGURES.

An embodiment of the invention will now be described by way of example,with reference to the accompanying drawings in which:

FIG. 1 is a block diagram showing a known technique for measuring RFpower.

FIG. 2 is a block diagram showing a known technique for calibration.

FIG. 3 is a block diagram showing an RF power measurement systemaccording to the invention.

FIG. 4 illustrates a calibration procedure used prior to measuring anunknown RF power.

DETAILED DESCRIPTION

Referring to FIG. 3, the unknown RF power to be measured is supplied toa first detector 11. With a switch 16 in position A, a digitiser 12gives a numeric output N_(11meas) corresponding to the voltage outputfrom the detector 11, which is then scaled in a numeric correction block13 to give the result. This part of the system is similar to the knownRF power measurement system shown in FIG. 1.

Referring again to FIG. 3, a RF power reference 14 is an oscillator witha well-defined output level and low harmonic content. This may beprovided, for example, by an oscillator followed by a divider circuit togive a 50% duty cycle, which in turn switches a well defined current onand off into a fixed load resistor. Harmonics from the resultingsquarewave output can be removed by a bandpass filter centred on theoutput frequency of the oscillator. With suitable selection ofcomponents and choice of operating frequency, the output level can bemade almost independent of temperature. In practice the operatingfrequency is usually chosen to be a low, fixed frequency to give thebest stability.

The output of the RF power reference 14 is connected to a seconddetector 15, whose characteristics are well matched to those of thefirst detector 11. To ensure that the characteristics of detectors 11and 15 remain well matched over a range of temperatures, they arethermally coupled to each other, for example by each being thermallycoupled to a common piece of metal 17. If the temperature of detector 11increases, the temperature of detector 15 also increases by the sameamount. In addition, both detectors are taken from the same manufacturedbatch.

With the switch 16 set to position B, the digitiser can measure theoutput from the detector 15 instead of the output from detector 11,giving a corresponding numeric output N_(15meas).

FIG. 4 shows the calibration procedure, which is performed at a fixedtemperature usually during manufacture. A known RF calibration power 19is connected to the detector 11 at a series of known frequencies Fncovering the range of interest. The switch 16 is set to position A, andat each frequency the output of the digitiser 12 gives a valueN_(11Fncal), which can be used together with the known RF calibrationpower to give the transfer function T_(11Fn) of the detector 11 anddigitiser 12 at the frequency Fn:

$\begin{matrix}{T_{11{Fn}} = \frac{N_{11{Fncal}}}{{RF}_{calpower}}} & {{Eq}\mspace{14mu} 1}\end{matrix}$

This series of numbers T_(11Fn) is retained in a stored number block 18.

Maintaining the same temperature conditions, the switch 16 is then setto position B.

The RF power reference 14 is measured via the detector 15, giving anoutput from the digitiser 12 which is recorded as N_(15cal). Since thereference is at fixed frequency, only one value of N₁₅ need be retainedin the stored numbers block 18.

Referring again to FIG. 3, the procedure for measuring an unknown poweris described. The unknown RF power to be measured is connected to thedetector 11, with the switch 16 set to position A. The reading from thedigitiser 12 is recorded as N_(11meas). The switch 16 is then set toposition B, and the RF power reference 14 is measured via the detector15, giving an output from the digitiser 12 which is recorded asN_(15meas).

The value of the unknown RF power can be calculated using the previouslystored transfer function T_(11Fn). However, the detector transferfunction varies with temperature, so the transfer function at thetemperature of measurement will be modified from the transfer functionat calibration by a temperature factor F_(temp):

$\begin{matrix}{{{Unknown}\mspace{14mu}{RF}\mspace{14mu}{power}} = \frac{N_{11{meas}}}{T_{11{Fn}}F_{temp}}} & {{Eq}\mspace{14mu} 2}\end{matrix}$

If the two detectors 11 and 15 are well matched, the transfer functionof the detector 15 will have changed in the same way as the transferfunction of the detector 11. The factor F_(temp) can be obtained bytaking the ratio of the measured and calibration readings of detector15:

$\begin{matrix}{F_{temp} = \frac{N_{15{meas}}}{N_{15{cal}}}} & {{Eq}\mspace{14mu} 3}\end{matrix}$

Substituting for Eq 3 in Eq 2, the unknown RF power is given by theexpression:

$\begin{matrix}{{{Unknown}\mspace{14mu}{RF}\mspace{14mu}{power}} = \frac{N_{11{meas}}N_{15{cal}}}{T_{11{Fn}}N_{15{meas}}}} & {{Correction}\mspace{14mu}{equation}}\end{matrix}$

By means of the correction equation above, the numeric block 13calculates the unknown RF power using the measured values N_(11meas) andN_(15meas) together with T_(11Fn) and N_(15cal) from the stored numbersblock 18. Although the calibration has been performed at only onetemperature, the measurement may be performed at other temperatures withonly a small loss in accuracy.

In an alternative implementation, the switch 16 may be omitted, and thedetector 15 may drive a separate digitiser.

1. A RF power measurement system comprising, a first detector forconverting RF power to be measured to a first measurement signal, areference source of RF power, a second detector, thermally coupled tosaid first detector, for converting RF power produced by said referencesource to a second measurement signal, and processing means fornumerically applying a predetermined correction factor to said first andsecond measurement signals obtained at the same temperature to derive avalue of said RF power to be measured.
 2. A RF power measurement systemas claimed in claim 1 wherein said predetermined correction factor isrelated to a predetermined transfer function of said first detector andbeing stored in a memory of said processing means.
 3. A RF powermeasurement system as claimed in claim 2 wherein said predeterminedcorrection factor is proportional to a ratio of said first and secondmeasurement signals obtained by a calibration process using an RFcalibration source in place of RF power to be measured.
 4. A RF powermeasurement system as claimed in claim 2 wherein said processing meansincludes a digitiser and said predetermined correction factor is relatedto a said predetermined transfer function of said first detector andsaid digitiser.
 5. A RF power measurement system as claimed in claim 4wherein said processing means includes switching means for selectivelyconnecting said digitiser to the output of said first detector and saidsecond detector.
 6. A method for measuring RF power comprising the stepsof: using a first detector to convert RF power to be measured to a firstmeasurement signal; using a second detector, thermally coupled to saidfirst detector, to convert RF power produced by a reference source to asecond measurement signal, and processing said first and secondmeasurement signals obtained at the same temperature by numericallyapplying a predetermined correction factor to said first and secondmeasurement signals to derive a value of said RF power to be measured.7. A method as claimed in claim 6 wherein said predetermined correctionfactor is related to a predetermined transfer function of said firstdetector and being pre-stored in a memory.
 8. A method as claimed inclaim 7 wherein said predetermined correction factor is proportional toa ratio of said first and second measurement signals obtained by acalibration process using an RF calibration source in place of RF powerto be measured.